How to get value from nextion to arduino

I am trying to make a pump pressure regulator with nextion as the pressure value input. how the value can be processed by arduino?

#include <SoftwareSerial.h>
#include <PID_v1.h>

SoftwareSerial mySerial(10, 11); /*Even though you can use the hardware serial port in this case I think it is better to
  leave the hardware serial open for debugging purposes*/

double Setpoint ; // will be the desired value
double Input; // photo sensor
double Output ; //LED

 double Kp=5, Ki=3.3, Kd=0.008;
//double Kp=3, Ki=1.2, Kd=0.01; 
// double Kp=1.2, Ki=0.19, Kd=0.001;
//create PID instance 
PID myPID(&Input, &Output, &Setpoint, Kp, Ki, Kd, DIRECT);
  
int rawValue; // A/D readings
int offset = 350; // zero pressure adjust
int fullScale = 9630; // max pressure (span) adjust
float pressure,mmhg; // final pressure
////////////////////////////////////// presure
int ardata[32];   // an array to store the received data
int count = 0;
int data;
int enA = 5;
int in1 = 4;
int in2 = 7;
int data_masuk;
boolean co=false;

void setup() {
  Serial.begin(57600); //open the hardware serial port

   Setpoint = 200;
  //Turn the PID on
  myPID.SetMode(AUTOMATIC);
  //Adjust PID values
  myPID.SetTunings(Kp, Ki, Kd);
  myPID.SetOutputLimits(-255, 255);
  myPID.SetSampleTime(3); 
 
  Serial.begin(9600); //open the hardware serial port
  mySerial.begin(9600); // set the data rate for the SoftwareSerial port
}

void loop() {
  
  rawValue = 0;
  for (int x = 0; x < 10; x++) rawValue = rawValue + analogRead(A0);
  pressure = (rawValue - offset) * 700.0 / (fullScale - offset); // pressure conversion
  mmhg=pressure*7.50062;
  delay(10);

  Input = mmhg;
  myPID.Compute();
  analogWrite(13,Output); //LED is set to digital 3 this is a pwm pin. 
   if(Output < 0)
  {    Output = 0;    }
  if(Output > 255)  
  {    Output = 255;  }
  digitalWrite(in1, LOW);
  digitalWrite(in2, HIGH);
  analogWrite(enA, Output); 

  Serial.print("t8.txt=\"");
  Serial.print(mmhg, 1); // one decimal places
  Serial.println(" mmhg ");
  Serial.print("\"");
  Serial.write(0xff);
  Serial.write(0xff);
  Serial.write(0xff);

  Serial.print(" INPUT ");
  Serial.print(Input);
  Serial.print(" OUTPUT "); 
  Serial.print(Output);
  Serial.print(" SET ");
  Serial.println(Setpoint);
//  Serial.println(" SET ");
  delay(10);
    
  
  String sendThis = ""; //Declare and initialise the string we will send
  if(mySerial.available()) {
        data = mySerial.read();
        delay(50);
        ardata[count] = data;
        if(ardata[0]==79 || ardata[0]==83)
        {                                       //   Serial.print("counter ");//   Serial.print(count);
          Serial.print(" Data masuk ");
          Serial.println(ardata[count]);
          count++;
                 if(count == 5 && ardata[0]==83)
                 {
                    if(ardata[2]==1)
                    { 
                     data_masuk = 256+ardata[1];//Serial.print("Lebih 255 :");                     //Serial.println(data_masuk);
                    }
                    if(ardata[2]==2)
                    {
                     data_masuk = 512+ardata[1];
                     Serial.print("Lebih 511 :");
                     Serial.println(data_masuk);
                    }
                      
                      data_masuk=ardata[1];
                      data_masuk=map(data_masuk,0,255,0,255);
                      Serial.println("Start");
                      digitalWrite(in1, LOW);
                      digitalWrite(in2, HIGH);
                      analogWrite(enA, data_masuk); 
                      delay(10); // wait for a second
                      count=0;
                 }

                   if(count == 5 && ardata[0]==79)
                   {
                        data_masuk=ardata[1];
                        digitalWrite(in1, LOW);
                        digitalWrite(in2, LOW);
                        analogWrite(enA, ardata[1]); 
                        delay(10); 
                        Serial.println("Stop");
                        count=0;
                   }

                   if(count > 5)
                   {
                    count=0;
                        Serial.println("Clear");
                      // wait for a second
                   }

        }
               }
  //delay(300); //Probably unneccessary, but I give the screen some time to respond
  data=data_masuk;
  sendThis = "n0.val="; //Build the part of the string that we know
  sendThis.concat(data); //Add the variable we want to send
  writeString(sendThis); /*Use a function to write the message character by character to the Nextion because
  mySerial.write(sendThis) gives you an error due to a datatype mismatch*/
 }


void writeString(String stringData) { // Used to serially push out a String with Serial.write()

  for (int i = 0; i < stringData.length(); i++)
  {
    mySerial.write(stringData[i]);   // Push each char 1 by 1 on each loop pass  
  }

  mySerial.write(0xff); //We need to write the 3 ending bits to the Nextion as well
  mySerial.write(0xff); //it will tell the Nextion that this is the end of what we want to send.
  mySerial.write(0xff);

}// end writeString function

Hello @14aulia ,

Can I suggest you start by reading this tutorial:Using Nextion displays with Arduino

Thanks